1. Field of Invention
The present invention relates to a scanning probe microscope (SPM).
2. Description of Related Art
A scanning probe microscope (SPM) scans a sample surface with a tiny probe, and detects an interaction between the tiny probe and the sample, so as to detect and to image a shape or physical quantity of the sample surface. One type of the SPM is scanning tunneling microscope (STM), in which the type of interaction measured is an electrical current flowing between the probe and the sample. Another type of the SPM includes atomic force microscope (AFM), in which the atomic force between the probe and the sample is monitored.
FIG. 3 shows a structure of the major parts of a conventional AFM (for example, referring to Japanese Laid-Open Application No. 2000-338027 ([0003], [0004], FIG. 7)). The conventional AFM includes a cantilever 34 with a sharp probe 33 at the front end, a displacement detection system for detecting a displacement of the cantilever 34, and a three-dimensional scanner (3D scanner) 31 for carrying a sample 32 thereon and moving the sample 32 along the X-Y-Z axis directions. When the front end of the probe 33 is relatively close (with a gap less than several nanometers) to the sample 32 mounted on the 3D scanner 31, an atomic force (attractive force or repulsive force) is present between atoms of the front end of the front probe 33 and atoms of the sample 32. On one hand, the scanner 31 is used for scanning the sample, such that the probe 33 moves relatively along the sample 32 surface within an X-Y plane. On the other hand, the scanner is used to move the sample in the z-axis direction such that a constant atomic force between the probe 33 and the sample 32 is maintained by feedback controlling the distance between the probe 33 and the sample 32 (the height in a Z-axis direction). The feedback in the Z-axis direction corresponds to a surface topography of the sample 32, and a three-dimensional image of the sample surface is obtained accordingly.
The displacement detection system is used to detect a displacement of the cantilever 34 in the Z-axis direction, which includes a laser source 35 for irradiating a laser beam to the vicinity of the front end of the cantilever 34, and an optical detector 36 for detecting the laser beam reflected by the cantilever 34 etc. By detecting the bending angle of the cantilever 34 according to an optical lever principle, the up and down movements of the cantilever 34 can be detected.
In the SPM, besides configuring the 3D scanner to carry the sample and to move the sample for making observations, a 3D scanner 30 can be also installed on the cantilever 34, and the probe 33 is moved by the 3D scanner 30 for making observations as shown in FIG. 4. In such a case, according to the signals from the displacement detection system, the displacement of the 3D scanner 30 in the Z-axis direction is feedback controlled in a manner that the atomic force between the probe 33 and the sample 32 is maintained constant, while a three-dimensional image of the sample surface is obtained by scanning along the sample 32 surface within the X-Y plane.
A scanner is generally a cylindrical body that includes piezoelectric elements, and moves freely in X-axis, Y-axis, and Z-axis directions, respectively, within a preset range under an externally-applied voltage. An observable range of the SPM is determined by the movable range (i.e., maximum scanning range) of the scanner in the X-Y axis direction. A scanner with a relatively large maximum scan range can ensure a relatively large observation range. To extend the maximum scan range of a scanner, the size of a scanner can be increased. On the other hand, to obtain an image with a high resolution, and to enhance the sturdiness of the whole device, the size of the scanner must be reduced. That is, it is generally difficult for a scanner with a relatively large maximum scan range to yield an image with a high resolution. Accordingly, it is difficult to ensure both a relatively large observation range and a high resolution at the same time.
Therefore, in the SPM, scanners with different maximum scan ranges must be used separately according to the observed targets or purposes. However, a problem in which the time-consuming practice of replacing scanners and the subsequent changing of settings will occur.
Also, in recent years, to correct the scale error caused by the non-linearity of the piezoelectric elements, a scanner incorporated with a position sensor has been widely used. The displacement of the scanner can be detected by the position sensor, and the displacement signal is fed back to the scanner's driving mechanism, thereby obtaining an image over a large area while maintaining the linear state. However, being incorporated with the position sensor, the device is further enlarged in size. Consequently, it is difficult to make observation at high resolution.
Also, the field of view is moved as time passes by, due to the heat drift of the sample in the SPM. Therefore, the extent that the field of view is moved by the heat drift, etc., is generally detected based on the obtained image and is corrected by the scanner, such that the observation can be resumed in the same field of view. However, when a small scanner is used for observation with a high resolution, and as the maximum scan range being smaller, the displacement of the field of view cannot be completely corrected during a long observation period. Occasionally, the desired area to be observed is moved outside of the field of view.